A NSW man was killed by his partner's lesbian lover because he was getting in the way of their relationship, a court has heard.

Tanya Louise Lane, 28, is on trial for the alleged murder of 33-year-old Steven Quire, whose decomposing remains were found in the Watagans National Park west of Newcastle in March 2010.

In the opening day of the murder trial it was alleged that Ms Lane shot Mr Quire in the chest or arranged for someone else to do so, so that she and the deceased's long-term partner, Renae Burns, 33, could be together.

"Tanya Lane and Renae Burns wanted to be together and Ms Burns' defacto was a major obstacle to this," Crown Prosector, Christopher Maxwell QC said in his opening address to the jury.

"Tanya Lane wanted him to be out of their life for ever," he said.

The court heard that Ms Lane became a regular visitor to the home Ms Burns and Mr Quire shared in the suburb of Rutherford in the months leading up to his death.

She had come to know the couple when she was in a relationship with Mr Quire's sister.

The already volatile relationship between Mr Quire and Ms Burns - which was marred by frequented heated arguments - became even more strained when the two women became more intimate.

"A witness saw the accused sit down next to Ms Burns and say to her 'let me show you how a girl can make you feel'," Mr Maxwell said.

"The deceased was angry with Tanya Lane and told him to get out of his house and never return."

It is alleged that over the coming weeks, Ms Lane asked a string of acquaintances whether they could help her get hold of a gun.

The court heard that she eventually paid a man $700 for a flare gun that could be modified with a special sleeve that allowed it to fire shot gun ammunition.

Mr Maxwell told the jury it was not known exactly what day the alleged murder took place, but that it took place between February 19 when the deceased was last seen by a neighbour and the March 13 when his remains were discovered by two men on a pig shooting trip.

The trial, before Justice Peter Hidden in the NSW Supreme Court, continues.